<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box {
      position: absolute;
      width: 100px;
      height: 100px;
      background: red;
      cursor: move;
    }
  </style>
</head>
<body>
  <div class="box" id="drag"></div>
  <script>
    window.onload = function () {
      var drag = document.getElementById('drag');
      // //点击某物体时，用drag对象即可，move和up是全局区域，
      // 也就是整个文档通用，应该使用document对象而不是drag对象(否则，采用drag对象时物体只能往右方或下方移动)  
      drag.onmousedown = function (event) {
        var event = event || window.event;  //兼容IE浏览器
        //    鼠标点击物体那一刻相对于物体左侧边框的距离=点击时的位置相对于浏览器最左边的距离-物体左边框相对于浏览器最左边的距离
        var diffX = event.clientX - drag.offsetLeft;
        var diffY = event.clientY - drag.offsetTop;
        console.log(diffY);
        if (typeof drag.setCapture !== 'undefined') {
          drag.setCapture();
        }
        document.onmousemove = function (event) {
          var event = event || window.event;
          var moveX = event.clientX - diffX;
          var moveY = event.clientY - diffY;
          if (moveX < 0) {
            moveX = 0
          } else if (moveX > window.innerWidth - drag.offsetWidth) {
            moveX = window.innerWidth - drag.offsetWidth
          }
          if (moveY < 0) {
            moveY = 0
          } else if (moveY > window.innerHeight - drag.offsetHeight) {
            moveY = window.innerHeight - drag.offsetHeight
          }
          drag.style.left = moveX + 'px';
          drag.style.top = moveY + 'px'
        }
        document.onmouseup = function (event) {
          this.onmousemove = null;
          this.onmouseup = null;
          //修复低版本ie bug  
          if (typeof drag.releaseCapture != 'undefined') {
            drag.releaseCapture();
          }
        }
      }
    }
  </script>
</body>

</html>